U.S. patent number 7,317,302 [Application Number 11/073,035] was granted by the patent office on 2008-01-08 for converter with feedback voltage referenced to output voltage.
This patent grant is currently assigned to National Semiconductor Corporation. Invention is credited to Michael John Collins.
United States Patent |
7,317,302 |
Collins |
January 8, 2008 |
Converter with feedback voltage referenced to output voltage
Abstract
A driver for a white LED string or a display is provided. The
driver includes a boost converter that is arranged to provide an
output voltage from a source voltage. Also, the driver includes a
sense resistor that is coupled between the output voltage and a
feedback voltage. The sense resistor is coupled in series with the
white LED string or the display. Further, the boost converter uses
the sense voltage across the sense resistor to regulate the output
voltage. In one embodiment, the boost converter includes a level
shifter that converts the sense voltage into a comparison signal
that is referenced to ground. In another embodiment, the converter
employs a reference voltage that is referenced to the output
voltage.
Inventors: |
Collins; Michael John
(Longmont, CO) |
Assignee: |
National Semiconductor
Corporation (Santa Clara, CA)
|
Family
ID: |
38893410 |
Appl.
No.: |
11/073,035 |
Filed: |
March 4, 2005 |
Current U.S.
Class: |
323/222;
323/285 |
Current CPC
Class: |
H02M
3/156 (20130101); G09G 2330/028 (20130101) |
Current International
Class: |
G05F
1/00 (20060101) |
Field of
Search: |
;323/222,285
;315/308,307,291,185R,193 ;363/39,259,260,21.17,59,60 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Mardiguian, Controlling Radiated Emissions by Design, 1992, Van
Nostrans Reinhold, first edition, pp. 96-99. cited by
examiner.
|
Primary Examiner: Vu; Bao Q.
Assistant Examiner: Behm; Harry
Attorney, Agent or Firm: Darby & Darby P.C. Gaffney;
Matthew M.
Claims
What is claimed is:
1. A boost converter for providing an output voltage from an input
voltage, comprising: a feedback circuit having at least a first
input and a second input, wherein the feedback circuit is arranged
to receive a reference signal at the first input; to receive, at
the second input, a comparison signal that is based on a feedback
voltage referenced to an output voltage rather than ground; and to
provide a control signal that is based, in part, on the reference
signal and the comparison signal, wherein the feedback circuit
includes at least one of an error amplifier or a comparator; a
driver circuit that is arranged to drive a switch circuit based, in
part, on the control signal such that the output voltage is
regulated based, in part, on the control signal; a reference
circuit that is arranged to provide the reference signal such that
the reference signal is referenced to ground; and a level-shift
circuit that is arranged to provide the comparison signal from the
feedback voltage such that the comparison signal is referenced to
ground, wherein the level-shift circuit includes: a first resistor
circuit, wherein the level-shift circuit is arranged such that a
first-resistor voltage across the first resistor circuit is
substantially equal to the feedback voltage, and further arranged
to provide a current responsive to the first-resistor current; and
an impedance current coupled between ground and the second input of
the feedback circuit, wherein the impedance circuit is arranged to
provide the comparison signal based, in part, on the first-resistor
current.
2. The boost converter of claim 1, further comprising: a pulse
width modulation control circuit that is arranged to provide a
pulse width modulation output signal such that a pulse width of the
pulse width modulation output signal is modulated based on the
control signal, wherein the feedback circuit is the error
amplifier, and wherein the driver circuit is arranged to drive the
switch circuit based on the pulse width modulation output
signal.
3. The boost converter of claim 1, further comprising: a pulse
frequency modulation circuit that is arranged to provide a pulse
frequency modulation output signal such that a frequency of the
pulse frequency modulation signal is modulated based on the control
signal, wherein the feedback circuit is the comparator, and wherein
the driver circuit is arranged to drive the switch circuit based on
the pulse frequency modulation output signal.
4. The boost converter circuit of claim 1, wherein the feedback
circuit is arranged to receive, at the second input, the comparison
signal, wherein the comparison signal is based on the feedback
voltage, and wherein the feedback voltage is referenced to the
output voltage rather than ground such that a voltage difference
between the feedback voltage and the output voltage is
approximately proportional to a load current.
5. A boost converter for providing an output voltage from an input
voltage, comprising: a feedback circuit having at least a first
input and a second input, wherein the feedback circuit is arranged
to receive a reference signal at the first input; to receive, at
the second input, a comparison signal that is based on a feedback
voltage referenced to an output voltage rather than ground; and to
provide a control signal that is based, in part, on the reference
signal and the comparison signal, wherein the feedback circuit
includes at least one of an error amplifier or a comparator; a
driver circuit that is arranged to drive a switch circuit based, in
part, on the control signal such that the output voltage is
regulated based, in part, on the control signal; a reference
circuit that is arranged to provide the reference signal such that
the reference signal is referenced to ground; and a level-shift
circuit that is arranged to provide the comparison signal from the
feedback voltage such that the comparison signal is referenced to
ground, wherein the level-shift circuit includes: a transistor
having at least an emitter, a collector that is coupled to an
output node, and a base that is coupled to the feedback voltage,
wherein the transistor is arranged to receive the output voltage at
the output node; a first resistor that is coupled between the
output node and another node; a translinear loop that is arranged
such that a resistor voltage at the other node is substantially
equal to a voltage at the emitter of the first transistor, wherein
the translinear loop includes a base-emitter junction of the
transistor; and wherein the first resistor is arranged to provide a
first-resistor current based on a voltage drop across the first
resistor; and an impedance circuit coupled between ground and the
second input of the feedback circuit, wherein the impedance circuit
is arranged to provide the comparison signal based, in part, on the
first-resistor current.
6. The boost converter of claim 5, further comprising a current
loop control circuit that is arranged to provide negative feedback
for the first-resistor current, wherein the current loop control
circuit includes at least part of the translinear loop.
Description
FIELD OF THE INVENTION
The invention is related to drivers, and in particular, to an
apparatus and method for driving a white LED string or display
employing a boost regulator that senses between Vout and FB to
maintain regulation.
BACKGROUND OF THE INVENTION
Light-emitting diodes (LEDs) may be used for lighting in portable
electronics applications, and the like. For example, white LEDs may
be used for back-lighting applications. Typically, several white
LEDs are coupled in series. A boost switching regulator may be
employed to provide a voltage across the white LEDs.
BRIEF DESCRIPTION OF THE DRAWINGS
Non-limiting and non-exhaustive embodiments of the present
invention are described with reference to the following drawings,
in which:
FIG. 1 illustrates a block diagram of a circuit for driving an LED
string or a display;
FIG. 2 shows a block diagram of an embodiment of a circuit for
driving an LED string or a display in which the feedback voltage is
referenced to Vout rather than ground;
FIG. 3 illustrates a block diagram of an embodiment of the circuit
of FIG. 2 in which the boost converter includes a level-shift
circuit;
FIG. 4 shows a block diagram of an embodiment of the circuit of
FIG. 3 in which an embodiment of the level-shift circuit is
schematically illustrated;
FIG. 5 illustrates a block diagram of an embodiment of the circuit
of FIG. 2 in which the reference voltage is referenced to Vout;
FIG. 6 shows a block diagram of an embodiment of the circuit of
FIG. 5 that includes a cut-off switch;
FIG. 7 illustrates a block diagram of an embodiment of the circuit
of FIG. 3 in which the boost converter is synchronously
rectified;
FIG. 8 shows a block diagram of an embodiment of the boost
converter of FIG. 3 in which the boost converter is
switched-capacitor based; and
FIG. 9 illustrates a block diagram of an embodiment of the circuit
of FIG. 3 in which the control circuitry includes a pulse frequency
modulation circuit, arranged in accordance with aspects of the
present invention.
DETAILED DESCRIPTION
Various embodiments of the present invention will be described in
detail with reference to the drawings, where like reference
numerals represent like parts and assemblies throughout the several
views. Reference to various embodiments does not limit the scope of
the invention, which is limited only by the scope of the claims
attached hereto. Additionally, any examples set forth in this
specification are not intended to be limiting and merely set forth
some of the many possible embodiments for the claimed
invention.
Throughout the specification and claims, the following terms take
at least the meanings explicitly associated herein, unless the
context dictates otherwise. The meanings identified below do not
necessarily limit the terms, but merely provide illustrative
examples for the terms. The meaning of "a," "an," and "the"
includes plural reference, and the meaning of "in" includes "in"
and "on." The phrase "in one embodiment," as used herein does not
necessarily refer to the same embodiment, although it may. The term
"coupled" means at least either a direct electrical connection
between the items connected, or an indirect connection through one
or more passive or active intermediary devices. The term "circuit"
means at least either a single component or a multiplicity of
components, either active and/or passive, that are coupled together
to provide a desired function. The term "signal" means at least one
current, voltage, charge, temperature, data, or other signal. Where
either a field effect transistor (FET) or a bipolar junction
transistor (BJT) may be employed as an embodiment of a transistor,
the scope of the words "gate", "drain", and "source" includes
"base", "collector", and "emitter", respectively, and vice
versa.
Briefly stated, the invention is related to a driver for a white
LED string or a display. The driver includes a boost converter that
is arranged to provide an output voltage from a source voltage.
Also, the driver includes a sense resistor that is coupled between
the output voltage and a feedback voltage. The sense resistor is
coupled in series with the white LED string or the display.
Further, the boost converter uses the sense voltage across the
sense resistor to regulate the output voltage. In one embodiment,
the boost converter includes a level shifter that converts the
sense voltage into a comparison signal that is referenced to
ground. In another embodiment, the converter employs a reference
voltage that is referenced to the output voltage.
FIG. 1 illustrates a block diagram of circuit 100. Circuit 100
includes battery 105, inductor L1, diode D1, boost converter 110,
output capacitor Cout, load 130, and sense resistor circuit R1.
Load 130 may be a white LED string or a display.
Load 130 is coupled between output node N1 and feedback node N2.
Sense resistor R1 is coupled between node N2 and ground GND.sub.1.
Sense resistor circuit R1 is series-coupled with load 130 so that
sense resistor circuit R1 receives load current Iload. Accordingly,
a voltage drop Vsense given by Iload*R1 appears across resistor
circuit R1. The voltage at node N2 is accordingly given by
Iload*R1. The voltage at node N2 is referenced to ground
GND.sub.1.
Also, boost converter 100 is configured to, in cooperation with
inductor L1, diode D1, capacitor Cout, and sense resistor circuit
R1, provide output voltage Vout from voltage VBAT, such that
voltage Vout is regulated based on feedback signal FB at node N2.
Signal FB is referenced to ground plane GND.sub.1, which is local
to boost converter 100. Boost converter 100 may be included in an
integrated circuit (IC).
However, circuit 100 is not suitable for certain applications. For
example, in certain applications, it may be preferable for the
display or white LEDs to be separate from the boost converter IC
and independent of grounding.
FIG. 2 shows a block diagram of an embodiment of circuit 200.
Circuit 200 includes battery 205, inductor L1, diode D1, boost
converter 210, output capacitor Cout, load 230, and sense resistor
circuit R2.
Ground GND.sub.1 is a local ground plane for boost converter 210.
In one embodiment, ground GND.sub.1 is a ground plane that is close
to battery 205 on a printed circuit board (PCB). GND.sub.2 is a
ground plane that is separate from ground GND.sub.1. In one
embodiment, GND.sub.2 is a ground plane on a PCB that is separate
from the PCB that GND.sub.1 is on. In another embodiment, GND.sub.2
is a separate ground plane that is connected at battery 205. In one
embodiment, load 230 may be on a flip phone or similar device,
having GND.sub.2 as a separate plane which is connected with wires
which may include resistances, inductances, and the like.
Accordingly, the voltage at GND.sub.2 may move relative to the
voltage at GND.sub.1.
Also, load 230 is coupled between output node N3 and ground
GND.sub.2. In one embodiment, load 230 is a string of series white
LEDs. In another embodiment, load 230 is a display.
Further, sense resistor circuit R2 is coupled between output node
N1 and feedback node N3. Sense resistor circuit R2 is
series-coupled with load 230 so that sense resistor circuit R2
receives load current Iload. Accordingly, a voltage drop Vsense
given by Iload*R2 appears across resistor circuit R2. The voltage
at node N3 is accordingly given by Vout-Iload*R2. The voltage at
node N3 is referenced to output voltage Vout.
Like circuit 100, circuit 200 is configured to, in conjunction with
other circuit elements, regulate output voltage Vout based on
feedback signal FB. However, in circuit 100, feedback signal FB is
referenced to ground. In contrast, in circuit 200, feedback signal
FB is referenced to output voltage Vout rather than ground.
By regulating output voltage Vout based on a feedback signal that
is referenced to Vout rather than ground, ground plane GND.sub.2
may be separate from ground plane GND.sub.1. This provides more
flexibility with the PCB layout, allowing the vendor to keep the
electronics located on one PCB while the display and/or white LEDs
are separate and away from the driver. Also, circuit 200 may
provide improved immunity to trace resistance and inductance found
in the display or LED bus line, GND.sub.2, or the connection at
node N3.
FIG. 2 illustrates an embodiment of circuit 200 in which an
asynchronous-rectified inductive-based boost converter is employed.
In another embodiment, boost converter 210 is a
synchronously-rectified converter. In another embodiment, boost
converter 210 is a switched-capacitor based boost converter.
In one embodiment, sense resistor circuit R2 is a single resistor.
In another embodiment, sense resistor circuit R2 includes two or
more resistors coupled in series and/or in parallel to provide a
total equivalent resistance.
In one embodiment, the components shown inside boost converter 210
in FIG. 2 are included together in an integrated circuit, and
components shown outside of boost converter 210 in FIG. 2 are
external to the integrated circuit. In other embodiments, some of
the components shown inside boost converter 210 may be external to
the integrated circuit and/or some of the components shown outside
of boost converter 210 may be internal to the integrated circuit.
For example, in one embodiment, sense resistor circuit R2 is
included in the integrated circuit, and in another embodiment,
sense resistor circuit R2 is external to the integrated
circuit.
FIG. 3 illustrates a block diagram of an embodiment of circuit 300.
Circuit 300 may be employed as an embodiment of circuit 200 of FIG.
2. Boost converter 310 includes level-shift circuit 350, feedback
circuit 340, control circuitry 360, driver circuit DRV1, and power
transistor M0.
In operation, level-shift circuit 350 provides comparison signal
Comp from signal FB by level-shifting signal FB such that signal
Comp is referenced to GND.sub.1. Feedback circuit 340 is arranged
to provide control signal Cntl from signal Comp and signal Ref.
Signal Ref is a reference signal that is referenced to ground.
Also, control circuitry 360 is arranged to provide signal DRVIN
from signal Cntl. In one embodiment, feedback circuit 340 is an
error amplifier, and control circuitry 360 is arranged to provide
signal DRVIN such that a pulse width of signal DRVIN is modulated
based, in part, on control signal Cntl. In another embodiment,
feedback circuit 340 is a comparator, and control circuitry 360 is
arranged to provide signal DRVIN such that a frequency of signal
DRVIN is modulated based, in part, on control signal Cntl (as
illustrated in FIG. 9 in one embodiment).
In either case, driver circuit DRV1 may be arranged to provide
switch control signal SCTL based on signal DRVIN. Additionally,
power transistor M0 may be arranged to operate as a switch circuit
that opens and closes based on switch control signal SCTL.
Although not shown in FIG. 3, in one embodiment, reference signal
Ref may be provided by a voltage reference circuit that is arranged
to provide signal Ref.
In one embodiment, reference signal Ref may be adjustable based on
an external signal that the brightness provided by load 330 is
adjustable. In other embodiments, the brightness may be adjustable
by adjusting the duty signal of an enable signal for boost
converter 310.
Level-shift circuit 350 may be arranged to operate as follows. A
resistor circuit (not shown in FIG. 3) in level-shift circuit 350
is arranged such that a voltage drop across the resistor circuit is
equal to the voltage drop across sense resistor circuit R2, and
further arranged to provide a current responsive to the voltage
drop across it. An impedance circuit (not shown in FIG. 3) coupled
between node N5 and GND.sub.1 receives the current and provides
signal Comp at node N5. An embodiment of level-shift 350 is
illustrated and described with reference to FIG. 4 below.
Although transistor M0 is shown as a MOSFET in FIG. 3, in other
embodiments, a type of transistor other than a MOSFET may be used
instead.
In one embodiment, the components shown inside boost converter 310
in FIG. 3 are included together in an integrated circuit, and
components shown outside of boost converter 310 in FIG. 3 are
external to the integrated circuit. In other embodiments, some of
the components shown inside boost converter 310 may be external to
the integrated circuit and/or some of the components shown outside
of boost converter 310 may be internal to the integrated circuit.
For example, in one embodiment, sense resistor circuit R2 is
included in the integrated circuit, and in another embodiment,
sense resistor circuit R2 is external to the integrated circuit. As
another example, in one embodiment, power transistor M0 is included
in the integrated circuit, an in another embodiment, power
transistor M0 is external to the integrated circuit.
FIG. 3 illustrates an embodiment of circuit 300 in which an
asynchronous-rectified inductive-based boost converter is employed.
In other embodiments, circuit 300 may be a synchronously-rectified
converter, as illustrated in FIG. 7 and discussed below in one
embodiment. In other embodiments, circuit 300 may be a
switched-capacitor based boost converter rather than
inductive-based, as illustrated in FIG. 8 and discussed below in
one embodiment.
FIG. 4 shows a block diagram of an embodiment of the circuit 400,
which may be employed as an embodiment of circuit 300 of FIG. 3.
Level-shift circuit 450 includes transistors Q0-Q7 and resistor
circuits R3-R5.
Transistors Q1, Q3, and Q6 are configured to operate as a current
mirror, and transistors Q4 and Q7 are configured to operate as
another current mirror. The base-emitter junctions of transistors
Q5, Q6, Q1, and Q0 are arranged to operate as a translinear loop so
that the voltage at node N4 is substantially equal to the voltage
at node N3. That is, the voltage at node N4 is given by
FB-VBE.sub.Q5-VBE.sub.Q6+VBE.sub.Q1+VBE.sub.Q0, or approximately
FB. Accordingly, the voltage across resistor circuit R3 is
substantially equal to the voltage across resistor circuit R2. The
current through resistor circuit R3 is substantially given by
Vsense/R3. Further, the current through resistor circuit R5,
current I1, is substantially the same as the current through
resistor circuit R3, so that the voltage drop across resistor
circuit R5 is substantially given by Vsense*(R5/R3). Accordingly,
the voltage at node N5 is substantially given by Vsense*(R5/R3),
referenced to ground GND.sub.1.
Additionally, resistor circuit R4 and transistors Q2, Q3, Q4, and
Q7 are optional circuit elements in level-shift circuit 450 that
are arranged to operate as a feedback loop to keep current I1 at a
substantially fixed value in spite of noise and other spurious
effects.
Transistors Q0-Q7 are illustrated as BJTs in FIG. 4. In other
embodiments, types of transistors other than BJTs may be employed.
Also, each of the resistor circuits R3-R5 may include one or more
resistors.
FIG. 5 illustrates a block diagram of an embodiment of circuit 500.
Circuit 500 may be employed as another embodiment of circuit 200 of
FIG. 2. Components in circuit 500 may be arranged to operate in a
similar manner to similarly-named components previously discussed,
and may operate in a different manner in some ways.
Feedback circuit 540 operates in a similar manner as discussed
above with regard to feedback circuit 340 of FIG. 3, except that,
instead of receiving ground-referenced signals, feedback circuit
540 receives signals referenced to output voltage Vout. Unlike the
reference circuit discussed above with regard to FIG. 3, reference
circuit 580 is arranged to provide signal Ref such that signal Ref
is referenced to output voltage Vout. Feedback circuit 540 is
arranged to receive signal Ref at its inverting input, and receive
signal FB as a comparison signal at its noninverting input.
Further, feedback circuit 540 is arranged to provide signal Cntl
based on signals Ref and FB.
FIG. 6 shows a block diagram of an embodiment of circuit 600.
Circuit 600 is an embodiment of circuit 500 of FIG. 5. Circuit 600
further includes transistor M2 and PMOS drive circuit 690.
Transistor M2 is arranged to operate as a cut-off switch responsive
to PMOS drive circuit 690. The cut-off switching is employed to
limit leakage currents and to enhance brightness control.
FIG. 7 illustrates a block diagram of an embodiment of circuit 700.
Circuit 700 may be employed as an embodiment of circuit 200, 300,
or 400 above. Boost converter 710 is a synchronously-rectified
converter which includes driver circuit DRV2 and power transistor
M1 instead of diode D1.
FIG. 8 illustrates a block diagram of an embodiment of circuit 800.
Circuit 800 may be employed as an embodiment of circuit 200, 300,
or 400 above. Components in circuit 800 may operate in a
substantially similar manner to similar-named components discussed
in previous figures, and may operate in a different manner in some
ways.
Circuit 800 further includes capacitors C1-C2 and diodes D2 and D3.
Boost converter 810 includes transistors M2 and M3, driver circuits
DRV3 and DRV4, oscillator circuit 870, error amplifier 840,
resistor circuit R5, and level-shift circuit 850. Boost converter
810 is a switched-capacitor based boost converter. Also,
level-shift circuit 850 is arranged to operate in a substantially
similar manner as described above with regard to level-shift
circuit 350 of FIG. 3. Driver circuit DRV4 is arranged to modulate
the on-resistance of transistor M3.
The above specification, examples and data provide a description of
the manufacture and use of the composition of the invention. Since
many embodiments of the invention can be made without departing
from the spirit and scope of the invention, the invention also
resides in the claims hereinafter appended.
* * * * *